Cancers may arise due to the normal expression of signaling proteins that sensitize specific cell types to specific oncogenic mutations. However, the means by which cell type-specific signaling pathways predispose to particular cancers is poorly understood. Retinoblastoma is a suitable model with which to address this issue, as the retinoblastoma cell of origin is exceptionally predisposed to initiate tumorigenesis in the event that the RB1 gene is inactive. Accordingly, this project aims to identify and characterize specific signals that sensitize the retinoblastoma cell of origin to the loss of Rb function. Preliminary Studies identified cone photoreceptor precursors (CPs) as a candidate retinoblastoma cell of origin and showed that human CPs are unique among retinal cells in that they prominently express the MDM2 oncoprotein and a thyroid hormone receptor isoform, TR2, that is also prominent in the precursors to Rb- deficient mouse pituitary tumors. They also revealed that MDM2 and TR2 are essential for retinoblastoma cell proliferation and survival, that MDM2 is needed to suppress the oncogenic stress response, and that TR2 is needed to sustain the expression of yet another protein that is essential to the development of Rb-deficient tumors, the F-box protein Skp2. To determine whether these CP features underlie retinoblastoma tumorigenesis, Aim 1 will define the roles of the cone circuitry in the response of primary human CPs to Rb depletion. Aim 2 will extend these in vitro studies by examining whether ectopic MDM2 expression in Rb- deficient mouse CPs - either from a cone-targeted transgene or from an intact human MDM2 locus - deregulates CP proliferation and elicits CP-derived retinoblastomas in vivo. Aim 3 seeks to define the role of TR2 in retinoblastoma cell proliferation and survival, and will specifically evaluate whether TR2 is needed to promote Skp2 expression and acts by antagonizing the related TR1 isoform. Aim 4 will evaluate whether TR2 contributes to the development of Rb-deficient mouse pituitary tumors in vivo, similar to its effects on human retinoblastoma cells in vitro, and will define the cellular effects of TR2 loss in the abnormally proliferating Rb-deficient cells that give rise to these tumors. Together, these studies will evaluate whether normal CP circuitry sensitizes cone precursors to the oncogenic effects of RB1 inactivation. The studies are likely to have an impact extending beyond retinoblastoma, by defining cell type-specific features that commonly collaborate with Rb loss to enable the development of Rb-deficient cancers.